Butterfly mystery exposed
Image: Neotropical butterfly Thecla opisena owes its iridescent green colour to the optical properties of millions of photonic crystals.
Using high resolution X-ray tomography, a team of international researchers has discovered exactly how the colours within the wing scales of a neotropical butterfly form.
The iridescent green colours on the scales of the Thecla opisena butterfly wings are produced by arrays of chitin photonic crystal nanostructures, known as gyroids, that only reflect specific wavelengths of light.
But while researchers have hypothesized how these chitin nanostructures polymerise and form within cell membranes during metamorphosis, surrounding optically thick and soft tissue has thwarted optical time-resolved in vivo imaging.
Given this, Professor Erdmann Spiecker from the Institute of Micro- and Nanostructure Research and Center for Nanoanalysis and Electron Microscopy, Friedrich-Alexander-Universität Erlangen-Nürnberg and colleagues used electron and X-ray imaging to unravel the butterfly research mystery.
Individual wing scales were lifted off butterfly wings and imaged using a Tescan MIRA3 field-emission SEM as well as a Zeiss Xradia 810 Ultra X-ray microscope.
Analyses revealed that the scales feature separated photonic crystal domains that increase in size from the base to the tip of the scales.
SEM image (right): Photonic crystal domains are isolated and increase in size from the base to the tip of the scale. Non-destructive X-ray tomography (bottom left): slice through a reconstructed tomogram shows relationship between the inner structure of the photonic crystals and their shape-giving membranes. [SEM: B. Wilts, NanoCT: CENEM/Zeiss-Xradia]
According to the researchers this is a so-far unique characteristic distinct from other butterflies; they believe this is a result of photonic crystal growth being time-frozen at different stages of the metamorphosis.
The researchers believe that the time-frozen sequence of these developmental stages indicates that crystal formation is a growth or extrusion process, and that the nascent chitin is extruded into a casting mould made of membranes.
"High-resolution X-ray tomography provided essential findings for a deeper understanding of the formation mechanisms... that current in vivo imaging methods cannot access," explains Spiecker. "The unique capability of X-ray tomography to analyse the 3D structure of entire wing scales was used to clarify where the chitin was finally extruded from."
And as Dr Benjamin Apeleo Zubiri adds: "The resolution of the reconstructed tomograms is so high, that we were able to clarify this question and, moreover, identify the chirality (handedness) of each individual photonic crystal."
The researchers believe that these intriguing 3D structures with unique optical properties may serve as prototypes for novel functional materials in applications such as photovoltaics.
Research is published in Science Advances.